The present invention relates to a prismatic sealed battery obtained by coupling a cover member to an opening portion of a prismatic outer case and welding the cover member and the opening portion, and to a method of manufacturing the prismatic sealed battery.
Recently, because of an increasing demand of portable OA devices and communication devices, there is a great demand for prismatic sealed batteries as power supplies. In particular, nickel metal hydride rechargeable batteries and lithium ion rechargeable batteries, the prismatic sealed batteries are small in size and can be mounted in the above devices with high volume efficiency.
In general, the prismatic sealed battery has an outer case 1A provided with an opening portion 3A, as shown in FIG. 1. A cover member 2A is coupled to the opening portion 3A of outer case 1A, and a coupling portion between these members is hermetically welded by a laser beam L.
When the laser beam L is radiated on the lateral side of the outer case 1A and scanned along the coupling portion between the outer case 1A and cover member 2A, the scan direction of laser beam L is controlled in the X- and Y-directions along the outer peripheral surface of outer case 1A in FIG. 1.
In a case where the laser beam L is incident on the lateral side of outer case 1A and scanned in the X- and Y-directions for welding, however, it is difficult to treat a corner portion R of outer case 1A and a corner portion r of cover member 2A. Specifically, if the laser beam L is scanned linearly in the X- and Y-directions, the laser beam L is not incident perpendicular to the outer periphery of the outer case 1A and cover member 2A at the corner portions R and r of outer case 1A and cover member 2A.
For example, when the radius of curvature of corner portion R, r is set at about 2.0 mm, an angle .theta. between the direction of radiation of laser beam L and a plane normal to a tangential line (including a surface to be welded) of the corner portion, R, r is 45.degree. at maximum. At this time, the area of radiation of laser beam L increases 1. 41 times (=1/cos 45.degree.).
Consequently, the radiation energy density (fluence: J/cm.sup.2) of laser beam L decreases by about 40%, compared to the case where beam L is incident perpendicular to the surface to be welded. In general, the allowance in a laser welding process with respect to a variation in the fluence is .+-.10%. Under the above condition, the depth of weld penetration also decreases and the strength of bonding deteriorates. As a result, cracks may occur at the welded portion.
In particular, in a case where the outer case 1A and cover member 2A is formed of aluminum or aluminum alloy in order to reduce weight, cracks will easily occur and the yield of manufactured products will considerably deteriorate. In order to achieve good welding, the condition, cos .theta.&gt;0.9, needs to be satisfied. Therefore, it is desirable to meet the condition, .theta.&lt;25.degree..
In a case where the outer case 1A and cover member 2A are formed of steel or stainless steel, the radius of curvature of the entire outer case including corner portions R, r is set to a small value, e.g. about 1.0 to 1.3 mm. Thereby, a deviation in angle of radiation of the laser beam L on the outer periphery of the corner portion R, r is decreased, and the strength of bonding is prevented from deteriorating.
In the case of using aluminum or aluminum alloy, however, if the radius of curvature of the corner portion R, r is decreased, the stress will concentrate at the corner portion R, r when the pressure in side the outer case 1A increases, and the rigidity thereof decreases. Since the strength of the aluminum or aluminum alloy itself is low, the radius of curvature of the corner portion R, r needs to be increased. Because of this, a deviation in angle of radiation of the laser beam L increases and the depth of welding penetration at the welded portion decreases. As a result, the welding strength decreases.
If the scan direction of the laser beam L is turned in accordance with the curvature of the corner portion R, r, such problem will not arise. However, if the scan direction of the laser beam L is turned along the corner portion R, r, the control for scanning the laser beam L becomes intricate and the cost of the scan device increases. Furthermore, a process time increases. Therefore, this measure is not practical.
On the other hand, the cross section of the outer case 1A of the prismatic sealed battery, which is parallel to the opening portion 3A, is formed prismatic (including a curve of the corner portion R), and the wall thickness (i.e. thickness of material forming the outer case) of a long-side portion 4A is equal to that of a short-side portion 5A.
If the wall thickness is not sufficient, the rigidity of the outer case 1A decreases. Thus, if a gas is produced by a chemical reaction of an electrolyte while a power generating element in the secondary battery is being charged, the internal pressure of the outer case 1 increases and, as indicated by solid-line arrows in FIG. 3, the inner surface of the long-side portion 4A of outer case 1A is pressurized in the direction of expansion and deformed.
Consequently, a bending moment is applied at the corner portions R, as indicated by broken-like arrows, and the short-side portions 5A are deformed inward by the bending moment and the entire outer case 1A is deformed.